Paddock Practices: Tips for tackling post-emergent weeds

Mixing and rotating herbicides as part of an integrated weed management (IWM) plan is key to achieving effective post-emergent weed control in crops and slowing further evolution of herbicide resistance in Western Australia.

The Australian Herbicide Resistance Initiative (AHRI), which has GRDC investment, says ensuring diversity in the use of herbicide modes-of-action (MOA) will minimise weed survival in the growing season and reduce longer-term development of weed ‘immunity’ to herbicides.

The Australian Herbicide Resistance Initiative’s (AHRI) Peter Newman, left, catches up on latest research into annual ryegrass resistance with AHRI’s Roberto Busi in the laboratory at The University of Western Australia. PHOTO: GRDC

The ‘mix and rotate’ message is one of the AHRI and industry-led WeedSmart initiative’s ‘Big 6’ IWM strategies for winning the battle against crop weeds. These also include:

For both of these options, rotate the mix to alternative MOAs the following year.

Peter says rotating herbicides across years remains highly important to IWM success, but research and experience is finding that mixing two herbicides together in one season is even more effective and can be sustainable.

He says this does not necessarily need to be a mix of two herbicides in the same tank, but could be an effective pre-emergent treatment followed by an alternative effective post-emergent application.

The aim is to get double knocks (two hits) on the same weeds with multiple herbicide MOAs.

It could be that this system might include trifluralin use each year. But experience in WA indicates this can stack-up if another effective post-emergent herbicide is also added to the mix and/or there is a second knock-down of weeds in the system, such as from crop competition, stopping weed seed set and/or effective harvest weed seed control.

Peter outlines more tips for herbicide use and IWM tactics in new GRDC Know More videos for managing annual ryegrass (Lolium rigidum) and wild radish in the western region. More results from her research can be found in a GRDC Grains Research Update paper and via a short GRDC video. Watch the first video 'Tactics to manage annual ryegrass' and the second video 'Tactics to manage wild radish.'

Know your herbicide resistance status

To evaluate the best post-emergent herbicide options, it is useful to know the resistance status of weeds on an individual property.

Commercial resistance testing can confirm the effectiveness (or lack of) of herbicides and establish a baseline of herbicide sensitivity to take the ‘guess work’ out of decision-making. This can be carried out with seed testing at the start of the season or after harvest.

Assessing the likely effectiveness of a post-emergent Group A or B herbicide after pre-emergent herbicides have dissipated is especially valuable for annual ryegrass and wild radish control.

At a state-wide level, AHRI conducts random herbicide resistance sampling every five years for the most commonly found weeds across the WA grainbelt.

The latest WA survey, led by AHRI researcher Mechelle Owen - who is a key researcher on the project, led by Charles Sturt University with GRDC investment - included samples from 509 cropping paddocks just before harvest in 2015.

These were tested using the highest recommended label rates of herbicides in 2016 and 2017 and some of the key findings for major weeds are outlined below.

For 338 populations of annual ryegrass:

96 per cent had resistant plants to the ACCase-inhibiting herbicide diclofop

83 per cent also had plants with resistance to sethoxydim

44 per cent had plants with resistance to clethodim at the higher rate

For the ALS-inhibiting herbicides, 99 per cent had sulfometuron-resistant plants and 97 per cent had plants resistant to imazamox + imazapyr

30 per cent had plants with some level of resistance to trifluralin

Only 2 per cent had plants resistant to atrazine

Some resistance to Boxer Gold® (prosulfocarb + S-metolachlor: lipid synthesis inhibitor) was detected - but this is still being evaluated

8 per cent had glyphosate resistance.

For 65 populations of wild radish:

88 per cent had resistant plants to the ALS-inhibiting herbicide chlorsulfuron

70 per cent had plants with cross resistance to the ALS herbicide mixture imazamox + imazapyr

61 per cent had plants resistant to 2,4-D

65 per cent had plants with diflufenican resistance (up from 50 per cent in the 2010 survey)

None had plants with resistance to glyphosate

Some had plants with resistance to atrazine (photosystem II inhibitor) but levels had not changed significantly since 2003.

For 97 populations of brome grass (Bromus):

About 15 per cent of brome grass populations displayed resistance to the sulfonylurea-type ALS-inhibiting herbicides sulfosulfuron and sulfometuron.

During the 2018 growing season, Mechelle will add 128 samples of wild oats (Avena) and 42 populations of barley grass (Hordeum) to the screening regime for weed herbicide resistance.

Post-emergent herbicide options

Peter Newman says the latest AHRI random sampling in WA highlights that, even though herbicide resistance is widespread for annual ryegrass and wild radish in this State, there are herbicide MOAs that remain highly effective.

Tips for annual ryegrass control

There is a suite of alternative pre-emergent options – such as Sakura® and Boxer Gold® – and, to date, no paraquat resistance has been detected in broadacre cropping in WA.

Dr Peter Boutsalis, of the University of Adelaide, says clethodim is another tool and its efficacy mainly depends mainly on application rate and environmental conditions. Research and experience show it is more effective on younger plants that are not exposed to cold/frosty conditions.

Dr Boutsalis says research into the value of rotating clethodim and butroxydim suggests butroxydim will sometimes kill plants that are moderately-resistant to clethodim and this could help drive down the weed seed bank.

He says herbicide resistance testing to determine the level of ‘dim’ resistance present can optimise which ‘dim’ treatments are effective.

To maximise control, it is advised to use quality glyphosate and paraquat products and surfactants at full label rates.

Dr Boutsalis says younger annual ryegrass plants are more sensitive, particularly at cooler temperatures. He says some glyphosate products are options for pre-harvest crop-topping in some crop types.

Tips for wild radish control

Velocity® (pyrasulfotole + bromoxynil, Group H/C) is highly effective for wild radish control in cereal crops and Peter Newman says no resistance to this MOA has been found in WA to date.

But to preserve its longevity, he says it is best to use Velocity® with other IWM strategies - especially harvest weed seed control.

Peter says research in the northern agricultural region in 2012 and 2013 also highlighted the value of treating wild radish with Velocity® (or other post-emergent herbicides) when small – about the size of the top of a beer can or coffee cup lid.

Carried out with GRDC investments by the Department of Primary Industries and Regional Development (DPIRD), Crop Circle Consulting and Landmark, this work targeted paddocks that had high populations of wild radish with multiple herbicide MOA resistance.

Peter says good control was achieved with application of Velocity® to small wild radish in a mix and rotate strategy with other post-emergent herbicides - in combination with tactics of good crop competition, stopping seed set and using harvest weed seed control.

Wheat yields improved by 22 to 36 per cent in the trials, worth an average $90-$130 per hectare at that time, when wild radish was controlled compared to untreated plots.

Tips for brome grass control

Brome grass is more effectively controlled in broadleaf crops or pastures than in cereal crops. DPIRD advises that pyroxsulam used post-emergence can be effective in the wheat phase. It says crop-topping using glyphosate in wheat or some pulse crops can also reduce brome grass seed set. Timing of the application is the key consideration, as brome grass matures a lot faster than some crops and will set seed before the crop can be legally sprayed.

Lupin, lentil and field pea crops typically mature early enough to kill many grass weeds, such as brome grass, using this tactic.

Tips for barley grass control

DPIRD advises that several ‘fop’ herbicides will provide good control of barley grass in broadleaf crops and sulfosulfuron is a good control option to use in the wheat phase (it is registered for suppression in wheat).

In cereals, an incorporated by sowing (IBS) mix of metribuzin and trifluralin is registered for barley and tolerant wheat varieties in WA.

DPIRD recommends using a break crop, such as lupin or canola, so triazines and Group A herbicides can be applied.

Where to from here

Peter says growers right across the WA grainbelt are keeping weed numbers relatively low by employing a range of weed control methods, including harvest weed seed collection and destruction that suppress big increases in the seed bank.

He says the ongoing challenge is to use a wide range of IWM tactics that will help achieve herbicide sustainability and optimise the productivity of cropping systems.

CAUTION: RESEARCH ON UNREGISTERED AGRICULTURAL CHEMICAL USE. Any research with unregistered agricultural chemicals or of unregistered products reported or linked to in this document does not constitute a recommendation for that particular use by the authors or the authors’ organisations. All agricultural chemical applications must accord with the currently registered label for that particular agricultural chemical, crop, pest and region.